This invention is related to hydrostatic fluid pressure and density measuring and monitoring sensors and systems for placement within the confines of a fluid medium such as liquid in a tank to measure the liquid depth and density within the tank at the location of the measuring device or sensor. The specific pressure sensor of this invention is in the category of pressure sensors that use a constant gas flow through the sensor device and relates the pressure of this gas flow to hydrostatic pressure of liquid at the level of the pressure sensor in the tank or container of liquid.
The broad basic sensing technique of measuring hydrostatic pressures at different depths in a liquid to determine depth and density has been used for many years and it provides a quite accurate measurement. This technique is usable to measure hydrostatic pressure in a liquid or in a gas. In using this measurement technique several constructions of specific pressure sensor devices have been built for use in specific applications. Depending upon the specific environment in which the pressure sensor must exist and operate the construction of the sensor device will vary considerably.
The specific application of this invention is measuring the hydrostatic pressure within a container of drilling fluid or mud of the character used with rotary drilling of oil and gas wells. The mud is basically a mixture of barite, water and other stabilizing elements. A feature of the drilling mud that renders it somewhat difficult to deal with is that it is thixotropic by necessity so that it will support suspended particles of cuttings once the circulation in an earth borehole has stopped. This feature of the drilling mud causes it to cake quite readily when its motion is stopped. This caking will take place not only in the earth borehole while being drilled but also in the tanks, containers, etc. for mixing and storing the drilling mud at the earth surface. This caking is accelerated when the mud is exposed to air and it begins to dry.
The pressure sensor of this invention is designed to be located in a drilling mud tank above the earth surface with the pressure sensor mounted in a fixed location in the tank. Because of the caking problem, any pressure sensor that is placed in such service will necessarily require periodic cleaning or removal of the mud cake so that it will operate properly and provide data within acceptable limits of accuracy. Because of the necessity for periodic cleaning the structure of this pressure sensor device must be quite rugged in order to withstand its sensing element being brushed, scraped or otherwise wiped clean of the cake material. In a drilling mud storage tank the uppermost pressure sensor would be located at a sufficient depth so as to remain submerged as the fluid level varies.
When these pressure sensors require cleaning they would need to be cleaned rather quickly so that drilling operations can proceed as a continuous twenty-four hour operation. The pressure sensors in such a tank would be raised to a point slightly above the surface of the fluid thus enabling a person to reach them. With the sensors raised a person can reach into the tank with a longhandled brush or scraping device to clean caked mud from the operative part of each sensor. It is to be expected that such cleaning would not normally be done in a particularly careful and sensitive manner and thus the pressure sensor structures must necessarily be quite rugged in their construction.
One such sensor known to be usable for this type environment is described in U.S. Pat. No. 4,111,047. This sensor construction has a pair of elongated flexible elastomeric membranes that lie in flush contact with each other so that gas can flow between them from one end of the sensor to the other while both pieces are surrounded by the liquid being tested. Pressure acting on the exterior of these flexible elastomeric members transmits liquid pressure in the container to the pressure in the gas. Such a construction is quite susceptible to physical damage when being cleaned due to the unsupported nature of the two members. Another embodiment shown in this patent replaces one of the flexible elastomeric members with a thin flexible metal member. This construction will be slightly more rugged but will however be subject to damage if the relatively thin metal member is bent or deformed by cleaning or handling.
Another construction known to be used in this type of pressure sensor has a pair of elastomeric membranes with one securely attached to a flat rectangularly shaped side of a support member and a second membrane positioned over the first membrane and sealed around the facing peripheral edge portions with no outer peripheral or edge protection. In this construction ports for gas communication between the membranes are located in a spaced relation on the support member and open through the first membrane that is securely attached to this support member. This permits flow from one port to the other between facing portions of the membranes in the limited area between the two ports. This construction while more rugged than the first described would still be vulnerable to damage if the bond between the membranes were to be exposed to the brush or scraping device during cleaning operations. In the event the flexible elastomeric membranes became unbonded this failure would necessitate replacement of the pressure sensor creating an inconvenience and expense in restoring this portion of the drilling mud handling system to its proper operation.
Further it is known that sensors designed to include facing flexible elastomeric members may fail in use if the constant gas flow between the membranes is temporarily interrupted while the sensor remains immersed in the fluid. This failure results from the facing membrane surfaces adhering to one another as a result of the now unopposed external hydrostatic pressure forcing the facing membrane surfaces into contact.
Pressure sensors in the category that use a constant gas flow through the sensor device and relate the pressure of this gas flow to the hydrostatic pressure of liquid at the level of the pressure sensor in the tank or container of liquid rely for their ability to operate with precision upon the degree with which the gas flow is maintained constant. Pressure sensors in this category known to be used in the applications described exhibit a relationship between the pressure of the gas and the flow rate of the gas. Extreme precautions are taken to maintain the gas flow rate at a fixed level.